The gloss, shade, color intensity, opacity, transparency and weathering resistance of paint coatings are influenced to a large extent by the particle size of the pigments in the paint surface.
One of the simplest and most commonly used methods of evaluating the quality of a pigment dispersion is the Hegmann test (DIN ISO1524). A so-called grindometer block is used for this test, comprising a longitudinally extending sample channel of steadily increasing depth which is filled with the paint to be investigated and then stripped and smoothed with a doctor blade (for example an Erichson grindometer with a channel length of 140 mm, a channel width of 12.5 mm and a channel depth at the deepest point of 15, 25, 50 or 100 .mu.m). When the paint surface is stripped with the doctor blade, the pigment particles in the V-shaped channel--beginning at the point of maximum channel depth--only slip through beneath the doctor blade to that point on the grindometer block where the channel depth corresponds to the particle size. In the adjoining section where the channel depth is smaller than the particle size, the pigment particles cannot slip through beneath the edge of the doctor blade. Now, the Hegmann test mentioned above is based on visual assessment of the number and position of the pigment particles visible on the grindometer surface after stripping with the doctor blade. At the point where particle size and channel depth are equal, the pigment particles become visible at the surface. Where the channel depth is greater, the particles sink to the bottom of the channel; where the channel depth is smaller, the particles are pushed along by the doctor blade. The position read off at this point characterizes the channel depth there and hence the particle size in .mu.m (see FIG. 1). The readings hitherto provided by the grindometer have the following further disadvantages:
the same sample is differently evaluated by different observers (subjective evaluation), PA1 the reading is influenced by the volatility of the solvent; the time between stripping and reading should therefore be at most only 3 s, PA1 evaluation of the pigment particles depends on the viewing angle and the light conditions. PA1 a) a light source with a focussing lens for producing a light spot on the paint surface to be investigated, PA1 b) a dark-field lens comprising an objective for measuring the scattered light emanating from pigment particles in the vicinity of the light spot and an aperture diaphragm for eliminating the light directly reflected at the paint surface, PA1 c) a photoreceiver connected to an evaluation circuit for detecting, recording and further processing the scattered light signals and PA1 d) a scanner for longitudinally scanning the sample channel. PA1 application and stripping of the paint surface on the grindometer block is followed by fully automatic, objective measurement; PA1 measurement of the particle size distribution accords very well with the conventional visual evaluation; PA1 the reproducibility of the apparatus is sufficient for all purposes; PA1 measurement of the particle size distribution is possible for all the usual colors.